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AOL has described a report which brands it as running the most zombie infected network on the internet as "meaningless" because it fails to take into account its large user base. Security firm Prolexic reports that AOL was the biggest single source of DDoS attacks over the last six months, accounting for 11.3 per cent of attacks in the US and 5.3 per cent worldwide.

But AOL spokesman Andrew Weinstein argues that its 21.7m US members meant it had 40 per cent of the US market, so figures from Prolexic that 11 per cent of hostile attacks monitored in the US can be traced back to AOL's network meant the ISP had a lower than industry average infection rate. Barrett Lyon, CTO of Prolexic, conceded that AOL had a point. "Our figures didn't take into account per-capita user base but regardless more computers on AOL are attacking online sites than from any other network. Just because a home user subscribes to a reputable brand doesn’t mean they’re safe from the online criminal fraternity."

DDoS attacks are often launched from machines compromised by malware such as Agobot and Spybot that turn them into drones on zombie attack networks (AKA botnets). Access to these botnets is sold online to spammers, cyber-extortionists or other ne'er do wells. When used in a DDoS attack, these compromised machines can 'flood' a network with fake packets, preventing legitimate traffic from accessing a site.

The disruption compromised Windows PCs cause to the wider internet is a recognised problem. Recently, internet firms banded to together in an industry wide push dubbed Operation Spam Zombies to wipe zombies off the net. AOL is a leading member of this initiative. It has also placed increased emphasis on consumer security in its recent software releases.

Prolexic, a 30 strong security start-up based in Miami, said its report highlighted a significant change in the way DDoS attacks are being coordinated. Instead of focusing on Layer-3 TCP attacks, hackers are increasingly trying "advanced full connection based flood" attacks. This trend allowed it to discount spoofing (forged destination) assaults before considering the origin of the attacks its customer face, according to Prolexic's Lyon. ®